Evidence is given that TL exhibits structural features that on present evidence are peculiar to TL among class I products of the H-2:Qa:Tla region of chromosome 17. First, the H chain of TL from normal thymocytes invariably appears on SDS-PAGE as a doublet of two components which, on the data obtained so far, differ in carbohydrate but not protein; TL+ leukemias, in contrast, have exhibited only a single H chain band. Second, TL+ thymocytes and leukemia cells of certain Tla genotypes produce additional TL products of higher molecular weights; combined genetic and biochemical studies of appropriate Tla hybrid mice indicate cis-restricted hemi-expression of one such high molecular weight TL product, apparently unaltered, in producer/nonproducer heterozygous thymocytes. For special application to TL primary protein structure, a strategy is described for the sequencing of an interior part or parts of the heavy chain with available N termini. The lack of Tlaa genomic clones and probes is a serious actual and potential handicap because Tlaa (strain A) has been and remains the most generally advantageous system in the study of Tla and its products. We have succeeded in subcloning a part of the BALB/c cosmid clone 6.3 that has sufficient Tla selectivity to screen for Tlaa genes in our A strain genomic library, which we are now doing. (CS)